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Galaxies &
Star Systems
Astronomy 2
Star Systems
 Our solar system only has one
star (our sun); however, most
are grouped together to groups
of two or more – called star
systems
 Star systems with 2 stars are
called double stars or binary
stars – 3 stars are called triple
stars
 Sometimes binary stars cannot
be seen from Earth – only one
star can be seen
Star Systems
 When one star hides another
star it is called an eclipsing
star
 Astronomers know there are
actually 2 stars by looking at
the effects of gravity
 Our solar system is not the
only solar system with planets
revolving around a star
 In 2000, astronomers
discovered a solar system
about 10.5 light-years away
with planets similar to our solar
system
Star Systems
 Scientists hypothesize that this
solar system might contain life
 Since this discovery scientists
have been sending out radio
waves and looking for radio
waves from unnatural sources
hoping to discover extraterrestrial life
 However, this could take many
years because radio waves
travel much slower than light
waves
Galaxies
 Galaxy: A group of
millions or billions of
stars, gas, and dust
held together by
gravity
 Our sun and all visible
stars are members of
our galaxy, the Milky
Way
Galaxies
There are 3 main
categories of
galaxies
Spiral galaxies
Elliptical galaxies
Irregular galaxies
Our Galaxy
 The Milky Way is visible
as a band of light
crossing the sky during
summer and winter
 Represents the
concentrated light of the
billions of stars in our
galaxy
 At night, we witness the
view from within our
galaxy
Milky Way Galaxy
 A disk of stars that
contains 100-200
billion stars
 100,000 light years in
diameter, about 3000
light years thick
 The center of the
Milky Way (nuclear
bulge), swells to
about 10,000 LY thick
Milky Way Diagram
http://zebu.uoregon.edu/~imamura/123/images/mw-schematic.jpg
Milky Way
Infrared Milky Way (COBE)
The view from
within, in IR
light
http://www.astro.psu.edu/users/niel/astro1/slideshows/class21/slides-21.html
Sun and Galaxy
 Sun located about 2/3
distance away from the
center (30,000 light
years)
 The entire Milky Way
rotates around its center
 Sun orbits the center of
the galaxy at 563,000
mph
 One galactic revolution
takes 220 million years
Sun’s Revolution
http://www.envirotruth.org/images/graphics/suns_path.jpg
Milky Way, a Spiral Galaxy
Spiral galaxy, most are concentrated in a
central nucleus
Spiral arms of stars wind outward from the
nucleus
M101
Milky Way Map
http://members.nova.org/~sol/chview/milkyway.jpg
Formation of the Milky Way
 Milky Way appears to
be about 13 billion
years old
 MW likely represents
one of the original
galaxies created after
the Big Bang
 Sun and planets are
4.5 billion years old
 Big Bang origin of
universe 13.7 bya
Elliptical Galaxies
 Elliptical Galaxies
Look like flattened
balls
Contains billions of
stars, but little dust
and gas
Due to the lack of dust
and gas new stars
rarely form
Most elliptical galaxies
contain only old stars
Irregular galaxies
Irregular galaxies
 Does not have a regular
shape
 One of the closest
neighboring galaxies to
the Milky Way is an
irregular galaxy
 It is about 160,000 light
years away
Star Clusters: Open Clusters
 A few dozen to thousands
of stars loosely bound
together by gravity
 Found mainly in the
galaxy’s disk and spiral
arms
 More than 1000 have
been discovered in the
Milky Way
 Young stars that recently
formed from nebulosity
Jewel Box open cluster (NGC 4755)
Star Clusters: Globular Clusters
 Huge, concentrated balls
of thousands to millions
of stars
 Found in galactic halo, a
spherical region centered
on the nucleus
 Contain the oldest known
stars
 About 150 globular
clusters have been
discovered
Omega Centauri, globular
cluster (NGC 5139)
Cluster Summary
Type
Stars
Location
Age
Open Cluster
Dozens to
thousands
(Population I)
Within galactic
disk
Globular Cluster
Hundreds to millions
(Population II)
Orbit galaxy in halo
Young,
Among oldest
thousands to
known stars (billions
millions of years of years)
Open Clusters
Pleiades—M45
M67
Which cluster is oldest? Note the star colors of the
main stars in each cluster
Globular Cluster M55
 Colors indicate
temperatures,
red (cool) to
blue (hot)
 The “turn off”
area on the
main sequence
represents the
cluster’s age
Between the Stars
 Space between the
stars in a galaxy is a
vacuum, or empty
space
 Also termed
interstellar medium
 Matter between the
stars is of gas and
dust
Between the Stars
 Interstellar matter 99%
gas, 1% dust
 Gas consists of 75%
hydrogen and 25%
helium
 Interstellar dust similar in
size to cigarette smoke
 In spiral galaxies, gas
and dust is concentrated
in the disk and spiral
arms
Pillars of
Creation
(M16)
•In this iconic
photo from the
Hubble Space
Telescope, a
small portion near
the center of M16,
the Eagle Nebula,
is revealed
•Note the pink,
newly formed
stars within the
nebula
Barnard’s S Nebula
A dark
nebula
represents
interstellar
dust that
blocks the
light of stars
from behind
http://www.astropix.com/HTML/D_SUM_S/B72.HTM
Horsehead
Nebula
Famous dark
nebula (B33)
located near the
Belt of Orion
Messier Catalog
All of the
Messier
objects are
shown in this
montage
M1 starts the
upper left,
M110 ends
the lower right
Other Galaxies
 Until 1924, Milky Way was the
“universe”
 Edwin Hubble proved that
galaxies other than our Milky
Way exist
 Hubble used Cepheid variable
stars to measure the distances
to galaxies
 Hubble also photographed
stars in the Andromeda
“Nebula”
 The universe contains an
estimated 100 billion galaxies.
Each of these galaxies
contains about 100 billion stars
Andromeda Galaxy
 Our nearest large
neighbor
 Similar in size and
shape to Milky Way
 Andromeda Galaxy is
visible to the naked
eye as a glowing spot
in the constellation of
Andromeda
 2.3 million light years
distant (wave!)
Andromeda Galaxy (M31, M32, M110)
Andromeda Constellation
Locate
M31 and
M33
Triangulum Galaxy (M33)
M33
The pink
glow of
emission
nebulas can
be seen in
this photo
Center of Virgo Cluster
M86
Virgo Cluster near M84, M86
Sombrero Galaxy (M104) from HST
Large-Scale Structure of the
Universe
Sheets &
Voids:
http://heasar
c.nasa.gov/d
ocs/cosmic/s
heets_voids.
html
http://www.astro.princeton.edu/~cen/PROJECTS/p1/DARKallz0.jpeg
Galactic Cannibalism
Galaxies can merge together to form
larger galaxies
Elliptical galaxies may be the result of
multiple galaxy collisions
NGC 2207 (left) and
IC 2163 (HST)